a) Field of the Invention
The present invention relates to a virtual display apparatus (VDA), with a light deflecting near-eye optic located in the peripheral field of view, for presenting to the eye a magnified virtual image of a miniature display when the viewer's gaze is directed towards the periphery. More particularly, the present invention relates to a head-mounted virtual display apparatus based on a non-cross-cavity optical configuration where a grouping of one, two or three light deflecting elements (IDEs) combine to redirect the light path from a miniature display towards the eye to provide “look toward” access to an inset virtual image, while simultaneously providing unobstructed forward vision. Active and passive alignment means, such as articulating connections and image warping electronics, allow for orthogonal alignment of the virtual image plane with the optical axis between the user's eye and the virtual image plane.
b) Description of the Prior Art
The head-mounted display (HMD) field has evolved on a number of fronts over the past 20 years. The earliest development by the military focused on wide field of view (FOV), see-through helmet-mounted displays for aircraft guidance and weapon aiming applications, in which the virtual image overlies the ambient environment. Since then development has included lightweight monocular HMDs for workplace wearable computer systems, binocular HMDs for full-immersion viewing of video and virtual reality applications, and various types of see-through displays for augmented reality applications.
Monocular HMDs are designed to provide access to electronic information while obscuring only a portion of the forward and peripheral fields of view (FOVs). A typical monocular HMD approach places the display and optics directly in front of one eye, such that the forward FOV of that eye is partially or fully occluded and the peripheral FOV of one or both eyes is partially occluded. The most common example of this type of monocular HMD is a boom style HMD, in which the viewable element (and often the display) is positioned in front of the face at the end of a cantilever arm. The main advantages of a boom style HMD include its relative simplicity (i.e., its one size fits all nature and minimal number of adjustments) and its construction flexibility, in that it can be added to a pair of spectacles or any head-borne structure, or can be constructed as a stand-alone headset. The disadvantages of a boom style HMD include a physical boundary that extends a distance from the face, occlusion of a portion of the forward FOV, and its suitability primarily for stationary activities due to vibration of the cantilever arm during user motion.
A second monocular HMD approach integrates the virtual display elements, in part or in full, into a pair of spectacles, with the aim of not significantly altering its form or weight. This approach allows the display and optics to be kept closer to the face, thus making it possible to limit the occluded FOV to one eye and, in some cases, to only a small portion of the peripheral FOV. The compact nature of a glasses-mounted display (GMD), however, generally requires a folding of the optical train, which increases the complexity of the construction.
In general, monocular HMDs can be categorized according to whether the optical train is an on- or off-axis configuration. In an on-axis optical configuration, the optical axis of each powered optical element coincides with the optical train axis or illumination path (with the exception of unpowered LDEs used to “turn corners”). In other words, no optics are “tilted” with respect to the optical train axis. Off-axis optical configurations, on the other hand, generally include at least one powered optical element whose optical axis is tilted with respect to the optical train axis. Off-axis optical configurations allow more compact constructions but typically suffer from higher levels of optical aberrations and geometric distortion.
Monocular HMDs can be further categorized according to the nature of the magnification system, of which there are two basic types: simple and compound magnification systems. A simple magnification system (or simple magnifier) is a single stage, non-pupil forming magnification system (i.e., a magnification system that does not form a real exit pupil), which is composed of either a positive refractive or reflective element, or multiple adjacent refractive elements with no spacing between them. A compound magnification system, on the other hand, is a pupil forming magnification system typically composed of two or more distinct stages. In a compound magnification system, the stage closest to the object is termed the objective or relay, while the stage viewed by the eye is termed the eyepiece or ocular. In a two stage compound magnification system, the objective forms an “intermediate” image (either real or virtual) that is the “object” projected virtually by the eyepiece. For the purposes of this invention, a third type of magnification system—termed a compound eyepiece—is defined as one in which multiple refractive and reflective elements (including the eyepiece) are in close proximity to one another with spacing between at least two of the elements. A compound eyepiece is effectively a single stage (pupil-forming) magnification system, which is typically located closer to the eye than it is to the display. Or, putting it another way, the distance between the display and the first magnifying element (or the “objective”) of the compound eyepiece is typically greater than the distance between the first magnifying element and the last element (or the “eyepiece”). For a compound magnification system the converse typically holds. For example, consider an HMD with a display located above the eye and a compound eyepiece located below the eye, which is formed from a single block of material and includes three magnifying surfaces: a refractive entrance surface, a reflective intermediate surface and a refractive exit surface. This device includes multiple spaced magnifying elements (so it cannot be categorized as a simple magnification system) and the distance between the entrance and exit surfaces (or the “objective” and “eyepiece” for comparison purposes) is less than the distance between the display and the “objective”. Thus, the magnifying power is not distributed throughout the optical train like a two stage, compound magnification system.
The design of an HMD involves two generally conflicting aims: achieving a high quality, computer monitor sized virtual image (i.e., a virtual image with a diagonal dimension of at least 10 inches and preferably 15 inches or greater) at a desired apparent image distance (such as a workstation distance of about 24 inches), and the desire for a compact, lightweight format. One method of balancing these aims is through the use of lightweight, reflective or light deflecting elements (LDEs), such as a mirror constructed from a plastic substrate and a reflective film. In addition, powered and unpowered LDEs may be used to increase magnification (the latter by increasing the optical train path length) and to distribute the weight of the optics more evenly about the head.
A monocular HMD for mobile activities must present a stationary virtual image to the eye during user motion. This requires that the support frame be stably secured to the head and that the display and optics be stably secured to the frame. Taking user comfort into account, the former requirement is best satisfied by a support frame in contact with both ears and the bridge of the nose; while the latter requirement negates the use of a relatively long, thin cantilever arm as the support structure for attaching the eyepiece to the frame, since this type of structure is susceptible to vibration during user motion. For safety and performance reasons, another key requirement for a mobile activity HMD is unobstructed forward vision.
For the purposes of the present invention, the head-mounted display field is further categorized according to: (i) whether the device is suitable for mobile activities; (ii) the optical configuration obstructs normal forward vision; and (iii) whether the optical configuration is a cross-cavity optical configuration (CCOC) or a non-cross-cavity configuration (non-CCOC).
As defined by Geist in disclosure Ser. No. 60/311,928, incorporated herein by reference in its entirety, a cross-cavity optical configuration is an optical configuration in which at least two elements of the optical train lie on opposite sides of the ocular cavity, such that when the system is properly aligned (using articulating alignment means), the light path crosses directly in front of a forward gazing eye. For example, in FIG. 1 a glasses-mounted virtual display (GMD) based on a cross-cavity optical configuration is represented, consisting of a single (horizontal) optical plane (termed the principal optical plane). Independent or simultaneous vertical translation of the display (70) and adjacent folding optic (27), combined with an extended light deflecting eyepiece (22), allows the optical train elements to be centered on the eye (50) to establish the principal optical plane.
In comparison, an example of a GMD based on a non-CCOC has the adjacent folding optic and the near-eye optic located in the normal reading glass location with the real image source assembly located near the cheekbone at the same horizontal level, such that the entire optical train is located below eye. A common feature of a mobile activity HMD based on either a cross-cavity optical configuration or a non-cross-cavity optical configuration, as defined by Geist, is that the light deflecting eyepiece may be positioned anywhere in the peripheral FOV.
More specifically, in U.S. Pat. No. 6,771,423, incorporated herein in its entirety, Geist defines a mobile activities HMD as an HMD with an unobstructed forward line-of-sight of at least 35° and an unshakeable head-borne mounting (i.e., a head-mounted support in contact with the bridge of the nose and at least two additional areas of the side(s) and/or back of the head, such that the resulting three contact areas provide a stable, unshakeable platform for the optical train). Suitable mobile activities head-mounted supports include, but are not limited to, conventional eyewear, goggles held in place with a strap or headband, and a headset style head-borne support in contact with one ear and/or the side of the head, in addition to the bridge of the nose.
A key factor in compact HMD designs is the level of image degrading factors. For the purposes of this invention, image degrading factors are divided into two general categories.
The first category of image degrading factors includes all types of geometrical distortions, including those inherent in most off-axis optical configurations. In general, geometric distortion represents the inability of the system to correctly map the shape of the object into image space (i.e., geometrical distortion is representative of a mapping error). In the case of conventional, symmetric distortion (commonly referred to as barrel and pincushion distortion), the image appears warped (or bowed) inwards or outwards. In the case of keystone distortion, a difference in path length from one area of the object to another results in a trapezoidal shaped image for a nominally rectangular object. Keystone distortion arises in off-axis projection systems and in optical systems when the optical axis of a powered optical element (or optic) is not perpendicular to the plane of the object (e.g., when the magnifying stage is tilted with respect to the display or vice versa). Keystone distortion is inherent in most off-axis HMD optical configurations, as are some higher-order, asymmetric types of geometric distortion.
In a paper entitled Image Plane Tilt in Optical Systems (SPIE No. 1527, Current Developments in Optical System Design and Optical Engineering, 1991), incorporated herein in its entirety, J. S. Sasian provides one of the most detailed analyses to date of geometric distortion in off-axis or non-axially symmetric optical configurations. Sasian derives a modified form of the Scheimpflug condition for a bilaterally symmetric systemAnu′ tan(θ′)−u tan(θ)=G+Wimage tiltin which An is the coefficient of image anamorphism; u and u′ are the angles of the marginal paraxial ray with respect to the optical axis in object and image space, respectively; θ and θ′ are the tilt angles of the object and image planes, respectively, relative to a plane perpendicular to the optical axis; G is a coefficient associated with the breaking of axial symmetry; and Wimage tilt is a coefficient associated with image plane tilt arising from optical aberrations. For an axially symmetric system with a tilted object plane, An=1 and G=0, and the regular Scheimpflug condition holds. When G is non-zero, tilting of the image plane may occur, even when the object plane is perpendicular to the optical axis. A relevant and interesting example with regard to the present invention is that of prism. While G=0 for both a prism and a flat mirror, Wimage tilt is non-zero for a prism due to coma (since the stop of the system is not located at the surface of the element), which leads to the familiar fact that image plane tilt is one-third of the prism angle. As further noted by Sasian, keystone or trapezoidal distortion is closely related to image plane tilt. The coefficient of keystone distortion is defined as:
  K  =            m      ⁢                                    tan            ⁢                                                  ⁢                          (                              θ                f                            )                                -                      tan            ⁢                                                  ⁢                          (              θ              )                                      f              =                  -                  A          n                    ⁢                                    tan            ⁡                          (                              θ                f                ′                            )                                -                      tan            ⁢                                                  ⁢                          (                              θ                ′                            )                                      f            in which m is the magnification, f is the front focal length, and θf and θ′f are the angles of tilt of the front and back focal planes, respectively.
The purely geometric nature of these types of image degrading factors allow them to be quantified and the display images predistorted (i.e., compensated electronically or computationally) in such a way as to cancel out the geometric distortion generated by the optic train. Presently a number of companies offer image warping digital signal processors for this purpose. For example, the sxW1-LX image processor from Silicon Optix is capable of predistorting images to correct for the aforementioned geometrical distortions. When applicable, this approach is particularly useful in HMD constructions since it allows the number of elements in the optical train to be kept to a minimum. For example, the sxW1-LX has been used to reduce the number of lenses in a Kaiser Electro-Optics helmet-mounted display from 21 to 6 per eye.
In practice, however, unless the distortion is of a fixed, unchanging nature, some means of adjusting the position and/or orientation of at least one optical train element is generally required to minimize or eliminate sources of geometric distortion in a multi-user HMD.
The second category of image degrading factors are those that cause a decrease in image sharpness or quality and include chromatic aberrations, astigmatism, coma and spherical aberrations, among others. This category of image degrading factors must be addressed through the use standard optical design techniques (which typically involves using multiple optical elements, surfaces and/or coatings to achieve a desired set of optical parameters, such as image magnification, exit pupil size, exit pupil location, etc.) while maintaining a level of image sharpness acceptable to the eye. For example, the off-axis optical configurations of most wide FOV, see-through HMDs suffer from a higher degree of coma, astigmatism and higher-order asymmetric distortion than a comparable on-axis optical configuration. The predominate image-degrading aberration of most off-axis optical configurations is third-order astigmatism, which, in the case of wide field of view HMDs, is typically minimized through the use of a toroidal reflective eyepiece.
Proper orientation and alignment of the observable virtual image plane is a key factor in user comfort during extended use of an HMD. Orienting a real image, such as a written document or computer screen, at a comfortable viewing angle is an every day activity. Quantitatively, orientation of the observable virtual image plane (VIP) is defined in terms of angles α and β (FIG. 2). Three groups of α and β values are pertinent to the present discussion. The first group corresponds to the case when the observable VIP is normal to the optical axis between the user's eye and the VIP, i.e., when α=β=90°. This corresponds to the image orientation when viewing an object at optical infinity and, for the purposes of this invention, is termed two-dimensional orthogonality. The second group of values of interest is when β differs from 90° and the image plane is thus tilted in an undesirable way. The third case of values is an acceptable deviation from two-dimensional orthogonality corresponding to a slight forward or backwards tilting of the observable VIP and is herein defined as one-dimensional orthogonality: β=90° and 120°≧α≧70°. Briefly summarizing, it is not generally acceptable to a viewer for β to deviate from 90°, but some deviation from two-dimensional orthogonality (corresponding to one dimensional orthogonality) may be acceptable to many users and may be preferable for certain user specific tasks.
It follows that a mobile activities HMD satisfying two-dimensional orthogonality (or one-dimensional orthogonality with a variable) generally requires one or more moveable/articulating connections (i.e., active alignment means) to align the optical train with the eye(s) of different users.
A number of boom-style or cantilever arm type HMDs have appeared in the prior art that do not obscure normal forward vision (such as U.S. Pat. No. 4,869,575 disclosed by Kubik) but are not suitable for mobile activities due to vibration of the cantilever arm during user motion. In addition a common disadvantage of this type of HMD is the inability to moveably and independently adjust the LDE that redirects the light path towards the user's eye (referred to herein as the near-eye LDE or near-eye optic).
Prior art based on an CCOC include Furness et. al. (U.S. Pat. No. 5,162,828), Heacock et. al. (U.S. Pat. No. 5,539,422), Bettinger (U.S. Pat. No. 4,806,011), Spitzer (U.S. Pat. No. 5,886,822), Holakovszky et al. (U.S. Pat. No. 5,129,716), Wells et. al. (U.S. Pat. No. 5,334,991) and Beadles and Balls (U.S. Pat. No. 5,648,789). Many of the embodiments of these inventions can be classified as mobile activities HMD. However, none of these inventions provide the alignment means necessary to orthogonally align the observable virtual image plane for different users when the near-eye optic is located in the peripheral FOV and normal forward vision is completely unobscured.
Kutz (WO 98/29775) discloses a mobile activities HMD based on a non-CCOC, wherein a pair of miniature displays and optical means are positioned above eye level. However, no aligment means are provided to establish one- or two-dimensional orthogonality for different users.